1. Field of the Invention
The present invention relates to a spark plug wherein a noble metal piece attached to a center electrode and a discharge face of a ground electrode are opposed to each other.
2. Description of Related Art
Recently, the attaching and detaching of spark plugs in automotive internal combustion engines are becoming difficult due to the increase of accessories, which gives rises to the need for more durable spark plugs that reduces or completely eliminates detaching works.
Also, the recent trend toward energy conservation asks for automotive internal combustion engines with better fuel economy and leaner burn, which necessitates spark plugs with higher ignition performance.
A conventional example to solve these problems is the spark plug shown in FIGS. 13 through 15 (Japanese Patent Examined Publication No.62-31797).
This spark plug has a center electrode 1 and ground electrodes 21 and 22 which are opposed to the tip 10 of the center electrode 1. The tip 10 of the center electrode 1 has a noble metal disk 3 at a position opposed to the spark points 219 and 229 of the ground electrodes 21 and 22.
The base 19 of the center electrode 1 is inserted and secured in the cavity of an insulator 4. The insulator 4 is inserted and secured in the cavity of the metallic housing 5, on which the ground electrodes 21 and 22 are formed. Sparks are induced at the spark gaps between the noble metal disk 3 and the spark points 219 and 229.
In the above spark plug, the contact area between the center electrode 1 and the noble metal ring 3 is large because the tip 10 of the center electrode 1 has a noble ring 3 over its entire circumference. Therefore, a heat stress is caused between the tip 10 of the center electrode 1 and the noble metal ring 3 due to the difference in the thermal expansion of the electrode material that forms the center electrode 1 and the noble metal that forms the noble metal ring 3.
This may cause a crack, peeling or fall of the noble metal ring 3. The peeled noble metal ring may even short-circuit the spark gap. Therefore, the spark plug can not obtain a prolonged service life in spite of utilizing the noble metal piece.
Another conventional example is the spark plug shown in FIG. 16 (Japanese Patent Examined Publication No.3-50396). This spark plug has an external electrode 25, a center electrode 1 having an aerial discharge gap Y between itself and the external electrode 25, an insulator 45 in which the center electrode 1 is inserted and secured, and a metallic part 55 which secures the base 46 of the insulator 45.
A noble metal part 30 is connected to the tip 10 of the center electrode 1. The tip 47 of the insulator 45 protrudes from the insulator 45 has a small diameter portion 471 with a smaller diameter than that of the base 46.
A gap Z exists between the spark point 26 of the external electrode 25 and the small diameter portion 471 of the insulator 45.
This spark plug is installed with a 14 mm thread.
However, the gap Z between the spark point 26 and the insulator 45 is designed to be about half the aerial discharge gap Y. Therefore, sparks may occur even at the gap Z if an engine having these spark plugs is initiated, when carbon or other matter adheres to the small diameter portion 471 of the insulator 45, or when the spark voltage at the aerial discharge gap Y increases depending on the engine operating conditions.
Therefore, a flame seed produced at the gap Z, which is narrower in effect, is more easily extinguished than that produced at the aerial discharge gap Y.
Also, the gap Z is not easily accessible to air-fuel mixture because it is surrounded by the external electrode 25 and the insulator 45. Furthermore, a spark at the gap Z less easily spreads to the rest of the combustion chamber than that at the aerial discharge gap Y, because the gap Z is located more distantly from the center of the combustion chamber than the aerial discharge gap Y when the spark plug is installed in the combustion chamber.
For these reasons, a spark at the gap Z not only shows very low ignition performance, but occurs frequently under ordinary engine operating conditions. Therefore, the ignition performance of the above spark plug remains insufficient.